Mann, described by a psychiatrist as “a remorseless, callous psychopath”,
slashed Harrison’s neck with a scalpel made from plastic cutlery and a razor
blade.

He also stabbed Harrison in the eye with a pen and, after he was dead, cut
open his abdomen.

Mann and Parr had planned to cut Harrison’s liver in half and share it, but
in the end could not bring themselves to do it, the court was told.

The case has echoes of the film Silence of the Lambs, in which cannibal
Hannibal Lecter boasts of eating a victim’s liver “with some fava beans
and a nice chianti”.

The judge, Mr Justice Openshaw, said both men presented a “gross and obvious
danger” as he handed them fresh sentences to be served on top of their
existing ones.

Mann, “one of the most dangerous men in the criminal justice system”, was
already serving life with a minimum term of 24 years after he climbed
through the window of a nursing home in Leicestershire in 2007 and killed
two vulnerable patients.

Psychiatrist Dr John Kent, who assessed Mann, said he was “a classic, cold,
callous and what I’d describe as homicidal psychopath” who had “compulsive
homicidal urges”.

He was given a life sentence with a minimum of 16 years to begin when his
current sentence expires, and will have served 40 years behind bars before
he can be considered for release.

Parr was jailed for life with a minimum term of 32 years, starting today.

He had been sentenced to life at Newcastle Crown Court in 2003 for attempted
murder after he tried to smother another patient at a hospital where he was
being treated.

Yesterday Mr Justice Openshaw said both men are likely to die in jail for the
“ghastly and gruesome” killing.

He said: “I can’t envisage circumstances in which either of them will ever be
released.

“They will face the fact that they may well end their lives in prison.”

Harrison, who was serving an indeterminate sentence for raping a 13-year-old
girl in Kendal, Cumbria, in 2009, was found dead by prison guards on the
morning of October 1, 2011.

Robert Smith QC told the court how Mann had calmly admitted butchering the
pervert.

The barrister said: “He told prison officer Graham in a calm manner that there
was a dead body in the cell.

“When asked why they did this, both of them shrugged their shoulders.

“Nathan Mann said he had chosen the deceased because he was arrogant and they
didn’t like him.

“He said they had planned it for a while and just fancied doing it.

“There was nothing to suggest a fight or even a scuffle had taken place.”

Mann said he tried to snap Harrison’s neck before proceeding with the gory
attack, the court heard.

Both men were arrested and taken to Durham police station, where Mann admitted
he and Parr had shared fantasies about killing.

“Nathan Mann told police that, after he had come to know his co-accused, he
told Parr he had thoughts about killing people and that Parr had told him he
thought the same way,” Mr Smith said.

“They both agreed that they would do it and that they would eat his liver
afterwards.

“They intended to remove his liver and cut it in half so they could both eat
it.”

Parr admitted murder, and the prosecution accepted Mann’s guilty plea to
manslaughter on the grounds of diminished responsibility.